The subject invention pertains to frequency detection of periodic signals in general and specifically to a digital technique especially suited for use with frequency tones such as used in connection with telephone pushbutton dialing.
The rapid supplantation of the venerable telephone rotary dial which generates dial pulses to indicate the digits of a called telephone number with the modern pushbutton keyboard which generates frequency tones to perform the same function has stimulated great interest in the frequency detection equipment required at the telephone switching exchange to detect and identify the transmitted frequency tones. As is well known, in the latter scheme each digit is represented by a unique pair of frequency tones, one selected from a low frequency group normally comprised of four different frequencies and the other selected from a high frequency group normally comprised of either three or four different frequencies. Since the actual frequency of each received tone is affected by the vagaries of the tone generator equipment in the telephone set itself and more importantly the telephone system linking the telephone set with the telephone switching equipment, it cannot be presumed that the received tones will have precise frequencies in accordance with established standards. This problem is obviated by establishing frequency bands providing frequency tolerance ranges into which the two frequencies which identify each digit must fall to be considered valid and the digit accepted.
At the inception of pushbutton dialing, the natural inclination was to perform the frequency detection function for identifying called digits, including establishing the frequency bands, with analog equipment such as LC filters. However, the benefits of reduced fabrication costs and equipment size made possible by the dramatically expanding digital technology field did not escape those involved with frequency tone detectors and so digital designs have been developed. These designs, however, do not appear to totally resolve two antithetical objectives in the detection process, namely, speed (the frequency tone is guaranteed to be present for a duration of only 40 milliseconds when the calling party merely taps the pushbutton of his telephone set as would normally be done) and reliability (assuring that only valid frequency tones are accepted and are not spurious noise or worse, "talk-off," which engenders audio signals which are sent by the telephone transmitter from the calling party who may happen to be speaking while also dialing and which are a particular problem since the frequencies employed for the frequency tones fall into the audible range). These former designs have been found to be either too exacting, in which case, good signals are rejected along with the bad thereby requiring telephone customers to be put to the inconvenience of redialing or, to the other extreme, too undemanding so that too many bad signals are accepted along with the good resulting in misconnections.
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved means for detecting and identifying the frequency of frequency tones such as employed in telephone pushbutton dialing.
It is a further object of the present invention to provide such a new and improved detection means which balances the need for speed and reliability in the detection process.
It is still a further object of the present invention to provide such a new and improved detection means employing counting techniques of simple design so as to lend itself easily to digital implementation.